B-cell lymphoma, unclassifiable, with features intermediate between diffuse large B-cell lymphoma and Burkitt lymphoma (iDLBCL/BL) is a newly categorized lymphoma in the WHO Classification of Tumors of Haematopoietic and Lymphoid Tissues v4. This lymphoma has characteristics of both DLBCL and BL, morphologically and genetically. We report a case with therapy-resistant iDLBCL/BL, which used to be categorized as BL in the WHO Classification v3. This case showed strong BCL2 staining, otherwise exhibiting features of BL. The case was treated with intensive chemotherapy containing high-dose methotrexate, cyclophosphamide, and cytarabine. However, soon after the chemotherapy, the lymphoma relapsed in the central nervous system and was resistant to whole-brain radiation therapy and allogeneic stem cell transplantation. The patient died 37 days after the transplantation. The clinical course was different from that of typical BL in terms of resistance to intensive chemotherapy, in agreement with the characteristics of iDLBCL/BL.